The overall decrease in sexual activity and pituitary-gonadal function observed with aging has led to the hypothesis that androgen deficiency is responsible for the decline in sexual behavior. Although testosterone is used in the treatment of sexual dysfunctions, there is a dearth of controlled, methodologically adequate studies on physically healthy individuals. The objective of this study is to investigate the behavioral and psychophysiologic effects of androgen administration on men aged 45 to 74 with inhibited sexual arousal or hypoactive sexual desire. Specifically, the central aims are to assess the effect of exogenous testosterone on (1) circulating hormone levels, (2) sexual behavior, affect and mood, (3) sleep and nocturnal penile tumescent (NPT) variables, and (4) erectile and subjective responses to controlled presentation of erotic stimuli. Sixty physically healthy men will receive testosterone enanthate at two dose levels in a double-blind, placebo controlled cross-over study design. During the initial phase, several sessions will be devoted to screening each couple, assessing psychosexual behavior, conducting a medical evaluation and blood chemistries, administering a battery of psychological instruments and recording erectile responses during sleep and in response to visual stimuli. The initial assessment will be followed by 18 weeks of treatment divided into three periods of six weeks in which the subject will receive each of the androgen doses and the placebo. Drug free intervals of four weeks will separate the drug and placebo periods. Interview rating will be conducted at weekly intervals at which time blood samples will be obtained for assessment of hormonal status. The administration of a battery of psychological instruments and recording of sleep/NPT and erectile responses to visual stimuli will be repeated at the completion of drug/placebo study periods. The effect of androgen administration on endocrine, psychologic, psychophysiologic and behavioral variables and the influence of age on these responses will be explored by a multifactor analysis of variance, correlation analysis and multiple regression techniques. Conceptually, this study based on a multidisciplinary framework may contribute to the understanding of the role of androgens on sexual function. At a practical level, it may generate important information on the efficacy of testosterone for the treatment of sexual dysfunctions in older men.